12/10/11 Miss Radford

Control of the cardiac cycleabout how the cardiac cycle is regulated and controlled.

WAL:

All

Most

Some

• What are “cardiac output” and “stroke volume” and how can these be calculated?

• How are the sinoatrial node, atrioventricular node and bundle of His involved in controlling the cardiac cycle?

• What is myogenic stimulation of the heart?

Your progress test on Cells will be on:

• Tuesday 3rd March

• Homework – revision and heart and lungs sections of unit 1 booklets (due in the following week) plus any left over disease sections from the first part of the booklet (due 10th March)!!

Today we are covering from the specification:

Starter – refresh your mind

• Complete the heart structure worksheet to recap how blood flows through the heart.

• Use the information sheet attached to help you.

2. Which chambers of the heart have thicker walls?3. Relate the thickness of the chambers to their

functions.4. What is the function of the valves? Label the valves

on the diagram above.5. What is the type of muscle that makes up the walls

of the heart? Why is it so special, and how does it maintain a constant blood supply?

6. Trance the pathway a drop of blood would take from the time it leaves the aorta, to the time it returns to the left ventricle ready to leave the aorta again, describing the chambers and vessels.

Definitions

• Heart rate: number of heartbeats per minute

• Stroke volume: volume of blood (cm3) pumped by heart in 1 beat

• Cardiac output: stroke volume multiplied by the heart rate gives the amount of blood (cm3) pumped by heart in 1 minute

Challenge: can you write these as numeric equations?

Heart rate = # beats minute

Stroke Volume = blood cm3

beat

Cardiac Output = blood cm3 X # beats = blood beats minute minute

Heart rate(beats/min)

StrokeVolume

(cm3/beat)

CardiacOutput

(cm3/min)

At rest 75 75

Duringexercise

180 120

5,625

21,600

What is the effect of exercise on cardiac output? Get out your

calculators!

The Cardiac Cycle• Cardiac Cycle - The events of one heart beat• One cycle – 0.8 second (72 cycles a minute)• 2 main processes – contraction and relaxation of the heart

muscle; • Diastole – lasts 0.5 seconds, represents the relaxation

phase, chambers fill with blood• Systole – lasts 0.3 seconds, represents the contraction

phase, blood pushed out of chambers/heart

The Heart’s Conduction System

• The heart is myogenic• This means it generates its own electrical

impulse• The impulse that it generates is spread

throughout the heart and causes it to contract • This is known as the cardiac impulse

The Spread of the Electrical Impulse

• The impulse starts in the SA Node (located at the top of the right atrium)

• Called the pacemaker

• The impulse travels through the atria walls

• This causes both atria to contract

• The cardiac impulse then reaches the AV node

• Also located in the right atrium

• The AV node helps delay the impulse to allow the atria to finish their contraction

• It then spreads the impulse down the bundle of His

• This is located in the Septum of the heart

• The bundle of His splits into left and right branches

• The impulse spreads around the ventricle walls through a network of purkinje fibres

• This causes both ventricles to contract

• The ventricles then relax • The cycle is repeated

with the next cardiac impulse

Cardiac Impulse

• http://video.about.com/heartdisease/Conduction-System.htm

Q. Describe how the sinoatrial node (SAN) and the atrioventricular node (AVN) control the increase in heart rate during exercise

• SAN initiates heart beat/sends impulses;• intrinsic/myogenic/pacemaker;• spread of impulses through atria;• atria contracts/systole;• impulse reaches AV Node;• Reduced delay of spread of impulses;• Bundle of His;• Purkinje fibres conducting impulses;• ventricular systole/contraction;• period of diastole/relaxation for filling;

Task – demonstrate your understanding

• Read pages 92 & 93 of your textbook.• Answer questions 1 to 5.